It's easy to analyze statistics to see who is performing well and who isn't. Box-score analysis is only part of the process when it comes to figuring out the best fantasy options.
Opportunity is also a factor, but it's more than just playing the most snaps or getting the most touches. Predicting how and when opportunities will come is also important. When Alex Collins fumbled near the goal line in Week 4, he wound up benched for the rest of the first half and may have cost himself future carries in scoring opportunities.
There's no doubt Kerryon Johnson has been the best running back for the Detroit Lions this season.